1. Field of the Invention
The present invention relates to the field of optical memories. More particularly, the present invention relates to a system and a method for recording a hologram in an optical memory.
2. Description of the Related Art
Volume holographic data storage has recently received much interest because such storage provides both high storage density and fast readout speed. Each hologram stores thousands of bits of data that are encoded as 2-D data pages of bright and dark pixels. In order to retrieve the stored data pages with low a bit-error-rate (BER), the pages must be carefully imaged from an input pixel array or spatial light modulator (SLM), through a small volume of the holographic storage material, and onto an output pixel array, such as a CCD detector. This requires a short focal-length lens system that is corrected for all aberrations (particularly distortion) over a large field, as well as a storage material having a high optical quality.
A conventional approach for overcoming this problem is to use a phase-conjugate readout of volume holograms. See, for example, F. Ito et al., Opt. Lett. 17, 215, 1992; M. C. Bashaw et al., Opt. Lett. 18, 2059, 1993; F. Zhao et al., Opt. Lett. 21, 1295, 1995; and J. J. P. Drolet et al., Opt. Lett. 22, 552, 1997, each of which is incorporated by reference herein. After recording the object light beam from an SLM with a reference light beam, the hologram is reconstructed using a phase-conjugate (time-reversed copy) of the original reference light beam. The diffracted wavefront then retraces the path of the incoming object light beam, thereby canceling out any accumulated phase errors. Such an approach allows data pages having high fidelity to be retrieved from storage materials fabricated as multimode fibers or using a low-performance lens, or even using no lens for an extremely compact system.
A drawback to using a conventional phase-conjugate approach has been in the need for many multiplexed pairs of phase-conjugate reference light beams. If the phase-conjugate reference light beams are formed from two carefully-aligned counter-propagating beams, then any deviation from a plane wavefront or misalignment of wavevectors will be transferred to the reconstructed object light beam. For angle-multiplexing, both of the counter-propagating beams must also have beam-steering. A phase-conjugate mirror (PCM), using external pump beams, self-pumped, or seeded by backscattering, can be used for creating a true phase-conjugate reference light beam. For each new reference angle or wavelength, however, the system must wait while the PCM reflectivity builds up, decreasing either readout or recording speed. If the PCM is used during storage (because recording speed is perhaps less critical), there will also be a loss of dynamic range as the strong forward beam erases previous holograms.
What is needed is a phase-conjugate volume holographic storage system that uses the same multiplexed reference light beam for both recording and readout operations and that overcomes the problems associated with conventional phase-conjugate volume holographic storage systems.